3.2. Leakage of lactate dehydrogenase activity
Faster stirring improves the efficiency of mixing, although it simultaneously increases the shear stress, eventually causing cell damage. Animal cells, which do not have cell walls, are fragile to physical stress. To check the damage during cell culture, we studied the leakage of lactate dehydrogenase (LDH) activity in the culture medium. As shown in Fig. 3C, in the bioreactor with 6FT or EE, leaked LDH activity rapidly increased after 120 h while reciprocal mixing had suppressed leakage. If the leaked LDH activity, during rotary mixing with EE, has to be suppressed as low as that in reciprocal mixing, stirring speed should be decreased to less than 100 rpm (data not shown). Stirring with EE at 100 rpm would induce insufficient mixing, resulting in poor growth and unsatisfactory gas exchange.